This invention relates to a method of bending a strand of steel in the vertical continuous casting process. Furthermore, the method relates to the use of corrugated rolls that work the surface of the steel strand.
The steel manufacturing practice for products which eventually were to be wrought or shaped by rolling to finished size, has been to cast the steel into ingots, a convenient intermediate form, which are further converted on primary mills to slabs or blooms suitable for finishing mill rolling. During the past thirty years, one of the most significant changes in the steel manufacturing process was the continuous casting of molten steel into shapes equivalent in section to conventional semi-finished shapes eliminating the ingot and primary mill stages.
Referring to FIG. 1, the basic construction of vertical continuous casting machines requires that molten metal be poured vertically into an oscillating water-cooled copper mold 10. With each oscillation of the mold 10 vertically an increment of solid steel 12 is attached to the existing strand shell 14 which is proceeding downward at the casting velocity. Each attachment of the initial ring of solid steel is recorded as an oscillation mark on the strand shell 14, with one mark made during each oscillation of the mold 10. These marks constitute stress risers and increase the sensitivity of the strand to creation of oscillator transverse cracks under tension stresses.
Referring now to FIG. 12, some early continuous casters produced a straight vertical strand but this design has given way to machines which requires less building height by converting the strand from the vertical to a horizontal path by either bending and unbending or casting into a curved mold (not shown) and restraightening. In the case of casters having straight molds 10, the strand is bent at some distance from the bottom of the mold into a curve configuration and at an additional distance bent in the reverse direction to emerge from the casting machine straight and horizontal. In the case of casters having curved molds (not shown), the strand emerges from the mold with curvature and requires only one straightening or bending operation to become flat and horizontal.
In existing continuous casters the forces for bending and restraightening are mechanical and are applied to the strand as it moves downward between a series of guide rolls positioned in a curved path. Typically, after the strand leaves the mold 10 it is cooled by water sprays (not shown) and travels vertically to allow for thickening of the solid shell 14 containing the internal molten metal 16. The strand then enters a driven pinch roll section 11 which withdraws the strand from the mold 10 and controls the speed of the line. Some of these pinch rolls are lightly knurled or serrated to assist in gripping the slab. The strand next enters the curved section in which the bending rolls 13 form the bend. The operation of these bending rolls is represented schematically by rolls 20, 22, 24 in FIG. 1. They are supported by back-up rolls as shown in FIG. 12 to prevent roll bending or flexing under the additional forces required to bend the strand.
After bending, the strand is guided through the remaining rolls 15 of the curved section, which includes additional sets of pinch drive rolls, to maintain curvature. As the strand emerges from the curved section, at floor level, it is bent in the reverse direction by the rolls 30, 32, 34 of the straightening machine 17 to become flat and horizontal. Shown schematically in FIG. 3, the rolls 30, 32, 34 of the straightening machine are larger in diameter than all of the previously described rolls in order to apply the additional forces required to bend the strand at reduced temperatures. Referring again to FIG. 12, a typical straightening machine has two pinch roll sets 31, 33 with openings set to the gauge of the strand at the entry and exit ends of the machine. Three intermediate rolls 30, 32, 34, positioned like an inverted triangle, bend the strand as a simple beam in the reverse direction of the strand curvature. The unbending load is applied by the two top rolls 30, 34 positioned approximately six feet apart and the bottom roll 32 located lengthwise midway approximately 3 feet between the two top rolls 30, 34. The top rolls 30, 34 have the adjustment capability to push the bottom of the strand below the roll pass line to allow for elastic springback of the strand.
The mechanical forces applied to the strand by the rolls 20, 22, 24 of the initial strand bender and the rolls 30, 32, 34 of the unbender, or straightening machine, treat the strand as a simple beam supported at the ends with a concentrated load applied at the center. This loading of the strand, or the bending moment, develops compressive stresses on the strand surface fibers at the point of center loading and tension stresses on the opposite strand surface. This type of loading is designed into the machine. In situations where the initial bender does not bend the strand to the desired curvature, the strand will further have tension stresses on either the top or the bottom surface while passing through the curved section of rolls, dependent upon the direction of out-of-curvature. This is because the curved section of rolls are trying to bend the slab to its desired radius. This additional stress can be crucial to subsequent crack failures. Although the straightening machine is primarily designed to remove upward curvature of the strand it has the capability to remove downward curvature by applying tension stresses to either or both surfaces.
The steel strand produced by the conventional continuous casting process has all the undesirable characteristics of steel castings such as a microstructure of large dendrites and large grain size in addition to internal stresses created during solidification as it is being pulled through the mold 10. Compounding the weakness of the steel are the effects of the oscillator mark stress risers resulting in a bending member highly sensitive to rupture under any source of tensile strain.
The process of casting into an oscillating mold 10 and bending and unbending the strand has resulted in many quality problems heretofore unknown to the steel producers. Aside from molten steel breakouts, which generally occur close to the bottom of the mold 10, the major quality problems are associated with strand surface cracking, both longitudinal and transverse, and entrapped slag inclusions. The most insidious of these defects are the small transverse cracks, invisible on the cold strand surface but visible on the final product. The occurrence of oscillator transverse cracks is a problem which has always accompanied use of the continuous casting process, particularly in certain high strength grades of steel. Of continuously cast steel slabs rejected, as much as 71% of these rejections have been caused by oscillator cracks. After the slabs are cast, they are rolled into plate. As much as 58% of plates produced in a run have been known to be rejected because of oscillator cracks. This can be a significant problem additionally because the oscillator cracks are not always visible to the naked eye. Detection requires dye penetrant inspection, and the probability of detecting all the oscillator cracks on a production basis is very remote. Various methods, metallurgical in nature, have been devised to attempt to deal with this problem, but none have been completely effective. Among these methods are placing restrictions on carbon content, modifying water sprays for strand temperature control, making additions of nickel, vanadium and titanium, revising casting temperatures and placing strict limitations on aluminum and nitrogen. Resorting to mechanical and hand scarfing, or grinding to remove these cracks is uneconomical and precludes direct rolling to save both energy and labor.
The present state of understanding on surface cracking is here summarized. Cracks appear on all grades of structural steels but are predominantly found on the microalloyed high strength low allow (HSLA) steels containing 1.0% manganese and 0.03% niobium (columbium) or vanadium. Cracks are the result of tension stresses applied to the strand surface during the bending or straightening operations. Cracks are generally found only on the top slab surface, i.e. the surface which is in tension when the strand is straightened. However, in severe cases they are found on both surfaces. Microscopic examination of fractures has established that cracking occurs at temperatures equivalent to caster bending and straightening operations; the hot ductility of fractures within the critical temperature range is inversely proportional to grain size; strain and strain rate exceeding that of caster bending and straightening promotes transition from intergranular to transgranular ductile fracture; and cracks initiate at the valleys of oscillator marks and propagate along grain boundaries. Steels exhibit a critical temperature range of low ductility between 1000.degree. C and 600.degree. C, known as the low hot ductility trough, aggravated by additions of niobium. Hot ductility within the trough temperature is further influenced by the strain and strain rate of deformation. The operating parameters of casters restrict the bending or straightening of the strand to the critical temperature range and the strain and strain rate of minimum hot ductility. In the case of the HSLA steels the deformation of bending or straightening promotes dynamic precipitation of carbonitrides such as niobium carbide and aluminum nitrides into and weakening of the grain boundaries. However, the small amount of deformation of bending is insufficient to cause recrystallization and grain refinement. It is noteworthy that precipitation of carbonitrides into the grain boundaries prior to the bending operation would improve ductility. The medium carbon (0.10% to 0.15%) steels exhibit additional crack susceptibility due to the formation of large columnar grains at the strand surface. This phenomenon is associated with the peritectic reaction during solidification of the strand shell in the mold. The columnar grains present grain boundaries particularly vulnerable to fracture by the lengthwise stress of bending.
To reveal these small tears, or oscillator cracks, producers have resorted to machine scarfing the entire surface of the strand preparatory to hand scarfing, or grinding the entire surface in conjunction with die penetrant testing to insure complete removal. This conditioning practice dictates that all steel grades having high "oscillator cracks" incidence be so processed.